Information readout from digital storage media with moving parts always involves the task of recovering the bit clock from the incoming time-continuous readout signal. Only with a reliably known bit clock can the readout signal be converted into a sequence of bits, and thus be ready for subsequent time-discrete, fully digital processing.
Additionally, whenever a specific storage media format foresees to group information into a repetitive pattern of data frames of predefined layout, also denoted as sync frames, the alignment of the data relative to this pattern has to be recovered. This is also denoted as sync frame alignment recovery. A related task, especially important during random access, is to verify from the read data which of the frames currently is being read, which is also denoted as readout address recovery. Sync frame alignment recovery and readout address recovery are made possible if e.g. every sync frame begins with a dedicated sync pattern and this sync pattern is additionally suffixed with an address information. The sync pattern together with the subsequent address information often is denoted as sync code. For the example of the CD format, the sync pattern is an 11T/11T signal, which is dedicated in so far as it surpasses, for the mark as well as the space, the 10T runlength limitation of pits containing ordinary data. Similar approaches are used for other optical media formats.
Additionally, whenever a storage media readout device is compliant to more than one storage media types or formats, the device has to recognize the media type at an early stage after media insertion, in order to be able to perform any subsequent data handling in accordance with the specification(s) relevant for that specific type of media.